How to work with three point lighting system?
One method is to set the pivot center type to 3D Cursor, place the cursor wherever you like, select a plane, duplicate it (Shift+D) and rotate (R) along the Z axis by 90 degress (you may type a rotation value using your numerical keyboard).


The other method is using a Mirror Modifier. Choose the axis you want your plane to mirror and input the name of your illuminated object in a window under Mirror Object:. Then apply the modifier.


Go to Edit Mode, select all mirrored meshes (A), then press P-->Separate-->By Loose Parts to make them separate objects. Now select one of them, snap the cursor to the center of it (Shift+S-->Cursor to Selected). Enter the Object Mode and press Shift+Ctrl+Alt+C-->Origin to 3D Cursor to place the origin point within. Repeat the same process with other planes.


You may now adjust the transformations of the planes changing the transformation orientation type to Normal (tap G, R or S twice to make it work).